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Shot peening is a commonly used surface treatment process used to improve the fatigue life of aircraft, automotive and other highly stressed structural components. This improvement is attributed to the formation of compressive residual stress on the surface layer of the material by the impingement of spherical media (shot). The compressive residual stress usually decreases the tensile stress created in the component by “in service” external forces and therefore increases the fatigue strength of the part. To quantify the improvement resulting from shot peening, the fatigue behavior of powder-forged connecting rods and laboratory test bars from the base material (2% copper steel), both in the stress-free (unpeened) and surface treated (shot peened) condition were compared. The fatigue data were correlated with the residual stress generated at the surface. The stress magnitude and depth were determined using x-ray diffraction analysis.